1. Field of the Invention
The present invention relates to a thin film transistor (TFT) liquid crystal display (LCD) and, more particularly to a gradation driving circuit which can output a large number of gray scale voltages in response to a minimum number of input voltages.
2. Discussion of Related Art
Generally, the TFT LCD module is a planar display, that is lightweight, thin, consumes little power, and has high contrast. Typically, the TFT LCD includes a back light which emits light through a matrix of pixels to the user. Each pixel includes three subpixels and associated color filters corresponding to the colors red, green and blue. Thus, for example, if a pixel were to emit blue light, the two sub pixels corresponding to the green and red filters are rendered opaque, while the sub pixel corresponding to the blue filter is made transparent. The human eye integrates light transmitted through the subpixels so that by selectively combining various combinations of the red, green and blue light, additional colors can be generated. Even more colors, however, can be displayed by further combining gray scales or gradations of each of the primary colors red, green and blue.
The gray scales are generated by supplying gray scale voltages to the individual sub pixels of the LCD, thereby causing the subpixels to have varying degrees of transmissivity. These gray scale voltages are output from a driver circuit capable of generating the requisite number of gray scale voltages at the appropriate levels.
TFT LCDs are commonly used in both audio video (A/V) and office automation (O/A) applications to display up to 512 colors using a 6-bit digital driver. As computers are increasingly being used in multi-media applications (transmission of both video and communication data) a wider range of colors are required. It has therefore been proposed that TFT displays generate up to 260,000 different colors by creating 64 gradations (or gray scales) for each of the primary colors red, green and blue. Although analog drivers, such as resistor ladders, can be used in certain A/V applications, a suitable driver for generating such a large number of colors in a high resolution O/A application has been difficult to realize.
A conventional gradation driving circuit of a liquid crystal display will be described below.
As illustrated in FIG. 1, the conventional gradation driving circuit includes a voltage source output part 1, a voltage source selection part 2 and an adder 3. The voltage source output part 1 outputs a plurality of different voltages. The voltage source selection part 2 selects the voltage source corresponding to the input data. The adder 3 receives and adds each voltage source output from the voltage source selection part 2, and outputs an appropriate gray scale voltage.
As shown in FIG. 2, the conventional output level selection circuit includes a plurality of source voltages V.sub.0 to V.sub.n (namely V.sub.0, V.sub.1, V.sub.2, V.sub.3, V.sub.4, V.sub.8, V.sub.16) each coupled to a respective one of a plurality of switches SW.sub.0 to SW.sub.n through one of resistors 24. Adder 3 is further provided to add various voltages supplied through selected ones of switches SW.sub.0 to SW.sub.n. Adder 3 can be realized with an operational amplifier 20 having its inverting input and output coupled to one another through resistor 22 and its non-inverting input coupled to ground. The voltage sum output of adder 2 corresponds to the gray scale voltage.
The operation of the conventional output level selection circuit will now be described. By way of example, if a gradation voltage corresponding to the 13th gray scale is to be selected, input data of 1011 (base 2), which equals 13 (base 10), is supplied to voltage selection part 2. The input data causes switches SW.sub.1, SW.sub.4, and SW.sub.8 to close, thereby coupling source voltages V.sub.1, V.sub.4, and V.sub.8, an input of adder 3. The 13th gray scale voltage is thus output as the sum of source voltages V.sub.1, V.sub.4, and V.sub.8.
FIG. 3 is a graph illustrating transmissivity of the LCD pixel (sub pixel) (T-V) as a function of applied gray scale voltage. As further shown in FIG. 3, due to the optical properties of the liquid crystal material, the relationship between applied gray scale voltage and transmissivity is not 15 linear, commonly referred to as the "gamma". Thus, the difference in potential between voltage V.sub.0 and V.sub.8 is not the same as the difference between voltage V.sub.8 and V.sub.16, for example. Accordingly, the interval between adjacent source voltages in the range of V.sub.0 to V.sub.8 is different than the interval between adjacent source voltages in the range of V.sub.8 to V.sub.16.
The conventional gradation driving circuit, however, outputs driving voltages that are spaced by the same interval across the range of V.sub.0 to V.sub.16. Thus, the conventional driving circuit does not compensate for the non-linearity of the transmissivity vs. gray scale voltage curve shown in FIG. 3, and does not provide "gamma correction".